The invention relates to a light-modulating cell comprising two substrates with a layer of electro-optical material in between, comprising a chiral nematic liquid crystal material which, in co-operation with the substrates, can assume a plurality of states, of which at least a focal conic state and a twisted planar state are stable in the absence of an electric field, said light-modulating cell being also provided with drive means for applying an electric field.
A light-modulating cell based on two (or more) stable states may be used in various applications, for example, in display devices where information written once should be maintained for a longer period (electronic newspapers, telephony, smart cards, electronic price tags, personal digit assistants, billboards, etc.), but also in known display application ns (instrument panels, monitors, television, etc.) or as an electronic shutter.
The invention also relates to a display device provided with such a light-modulating cell.
A light-modulating cell of the type described in the opening paragraph is disclosed in U.S. Pat. No. 5,453,863. The light-modulating cell described in this patent has a plurality of stable states, namely light-scattering states, which correspond to the focal conic state of a layer of liquid crystal material, and a reflecting state which corresponds to the twisted planar state of the layer of the liquid crystal material. In the light-scattering states, the light is scattered to a greater or lesser extent, dependent on the grey level. In the twisted planar state, the liquid crystal molecules are located substantially parallel to the substrates in the form of a helix structure with a pitch P (the axis of the helix is perpendicular to the substrates). In the focal conic state, the helix structure is interrupted, but groups of molecules still have helix structures, preferably with the axes of the helices being mainly parallel to the substrates, but these may also be located at different angles.
For a satisfactory light scattering in the light-scattering state, the light-modulating cell should be sufficiently turbid. This means that with admixtures, or the light-modulating cell must have a sufficient thickness. The latter means that the electric voltages to be used for switching become considerably higher, certainly too high for a number of said (portable) applications. Moreover, the switching time increases with an increasing thickness of the light-modulating cell.
It is an object of the present invention to obviate one or more of said drawbacks as much as possible. To this end, a light-modulating cell according to the invention is characterized in that the optical rotation of the layer of electro-optical material in the focal conic state and in the twisted planar state has different values, and the light-modulating cell comprises means for enabling detection of the difference of optical rotation.
The difference of optical rotation is preferably at least 20 degrees.
A further preferred embodiment is characterized in that the layer of electro-optical material is substantially transparent in the focal coni state, and a polarizer is present on at least one of the two sides of the light-modulating cell.
xe2x80x9cSubstantially transparentxe2x80x9d is understood to mean that light which is incident on the light-modulating cell is mainly scattered in the forward direction and reflected as little as possible. At least 25% and preferably 50% of the beam of incident light is passed.
The invention is based on the recognition that optical rotation takes place outside the optical band where reflection predominantly occurs in a chiral nematic or cholesteric layer of liquid crystal material, notably in the twisted planar state. In the focal conic state, this rotation is relatively small so that discrimination between two polarizers (in transmission) is possible between a substantially (light-)transmissive state and a substantially (light) non-transmissive state. In practice, switching takes place between two states each having a different optical rotation.
In order that reflections of ambient light have no detrimental effect, the twisted planar structure has such a pitch length that light outside the visible spectrum is reflected. Preferably, use is made of reflection in the infrared range (wavelengths between 700 and 3000 nm and preferably between 700 and 1500 nm) because it has been found that the switching voltage decreases with an increasing pitch.
A preferred embodiment of a light-modulating cell according to the invention is characterized in that the light-modulating cell is present between two polarizers whose directions of polarization have a mutual angle of between 30 and 140 degrees, preferably approximately 90 degrees. In this case, the contrast is optimal.
These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiments described hereinafter.